Controlled rectifiers (SCR'S) can be used for controlling the supply of current from a supply source to a load device, but a practical difficulty that arises in such circuits, is due to the fact that the SCR, once triggered, will remain conducting until the anode voltage or current has been reduced to a low value.
It is for this reason that SCR's are used in circuits, particularly alternating current circuits, where the supply voltage periodically drops to a low value to render the SCR non-conducting. This consideration militates against the use of SCR'S in controlling d.c. supply load devices.